Semiconductor light-emitting device and method for fabricating the same

ABSTRACT

Projections/depressions of a two-dimensional periodic structure are formed in a p-GaN layer ( 4 ) such that the period of the projections/depressions is 1 to 20 times the wavelength of light radiated from an active layer ( 3 ) in a semiconductor. As a result, a diffractive effect achieved by the projections/depressions of the two-dimensional periodic structure change the direction in which the light radiated from the active layer ( 3 ) travels. If the projections/depressions are not provided, light at a radiation angle which satisfies conditions for total reflection at the interface between a semiconductor device and an air cannot be extracted to the outside of the semiconductor device so that the light emission efficiency of the device is low. By contrast, the projections/depressions as formed with a period according to the present invention diffract the light at an angle which does not cause total reflection so that the efficiency with which the light is extracted to the outside of the semiconductor device is improved exponentially. This improves the light emission efficiency of the device.

BACKGROUND OF THE INVENTION

The present invention relates to a semiconductor light-emitting deviceand, more particularly, to a semiconductor light-emitting device made ofa nitride compound semiconductor such as GaN, InGaN, AlGaN, or the like.

In recent years, the use of a nitride compound semiconductor representedby GaN has allowed a high-intensity emission of light ranging in colorfrom ultraviolet to blue and green, which had been impossible thus far.As a result, light-emitting devices using nitride compoundsemiconductors, such as a light-emitting diode (LED) and a semiconductorlaser, have been developed vigorously. Since an LED is easier tofabricate and control than a semiconductor laser and longer in lifespanthan a fluorescent lamp, an LED using a nitride compound semiconductoris considered to be promising as a light source for illumination.

A description will be given herein below to an example of a conventionalnitride compound semiconductor LED. FIG. 16 is a perspective viewshowing a structure of the conventional nitride compound semiconductorLED disclosed in FIG. 10 of Prior Art Document 1 (Japanese Laid-OpenPatent Publication No. 2000-196152).

In the conventional LED, as shown in FIG. 16, a sapphire substrate 101,a GaN buffer layer (not shown), an n-type GaN layer 102, an InGaN activelayer 103, and a p-type GaN layer 104 are formed successively by crystalgrowth and a trench 108 for exposing the n-type GaN layer 102 as thebottom surface thereof has been formed by partly etching away the InGaNactive layer 103 and the p-type GaN layer 104. An n-side electrode 106is provided on the portion of the n-type GaN layer 102 exposed as thebottom surface of the trench 108, while a p-side transparent electrode105 is provided on the p-type GaN layer 104 and a p-side bondingelectrode 107 is provided on a part of the p-side transparent electrode105.

The following is the operation of the LED. Holes injected through thep-side bonding electrode 107 expand laterally in the p-side transparentelectrode 105 to be injected into the InGaN active layer 103 from thep-type GaN layer 104. On the other hand, electrons injected through then-side electrode 106 are injected into the InGaN active layer 103 fromthe n-type GaN layer 102. Light emission occurs upon the recombinationof the holes with the electrons in the InGaN active layer 103. The lightis emitted to the outside of the LED through the p-side transparentelectrode 105.

However, such a conventional structure has the problem of low lightextraction efficiency. The light extraction efficiency is the ratio oflight generated in the active layer and emitted from the LED into an airto all the light generated in the active layer. The cause of the lowlight extraction efficiency is the refractivity of a semiconductor whichis higher than that of the air. As a result, the light from the activelayer is totally reflected by the interface between the semiconductorand the air and confined to the inside of the LED. For example, therefractivity of GaN is about 2.45 when the wavelength of the light is450 nm so that a critical refractive angle at which total reflectionoccurs is as small as about 23 degrees. That is, light radiated from theactive layer at an angle larger than the critical refractive angle interms of a normal to the interface between the semiconductor and the airis totally reflected by the interface between the semiconductor and theair so that the light emitted from the active layer and extractable tothe outside of the LED accounts for only about 4% of all the lightemitted from the active layer. Thus, the conventional LED using anitride compound semiconductor is low in external quantum efficiency(the ratio of light that can be extracted from the LED to currentssupplied to the LED) and has the problem of power conversion efficiency(the ratio of a light output that can be produced to all the suppliedpower) lower than that of a fluorescent lamp.

As a solution to the problem, a technology which formsprojections/depressions at the surface of the LED has been proposed, asdisclosed in FIG. 5 of Prior Art Document 1. FIG. 17 is a perspectiveview showing the structure of the conventional nitride compoundsemiconductor LED disclosed in FIG. 5 of Prior Art Document 1.

In the structure shown in FIG. 17, projections/depressions each of aspherical lens structure have been formed in the p-type GaN layer 104.The structure has the possibility that, even though the angle formedbetween emitted light and a normal to the interface between the flatportion of the p-type transparent electrode 105 and the air is largerthan the critical refractive angle, the angle of incidence of the lightbecomes smaller than the critical refractive angle if it is incident onthe portion provided with the projections/depressions. Accordingly, theprobability that the light generated in the active layer is emitted tothe outside of the LED without being totally reflected increases and theexternal quantum efficiency is thereby improved.

However, the technology for improving the light extraction efficiencybased on the principle proposed in Prior Art Document 1 isdisadvantageous in that, because the angle of incidence of lightsensitively varies in response to the configuration of a projecting anddepressed surface, the design of the projecting and depressed surface isextremely difficult and characteristics are unstable due to sizevariations during the fabrication of devices. The technology is alsodisadvantageous in that, though the projections/depressions to be formedshould have a depth of about several micrometers to improve the lightextraction efficiency, processing is difficult because of the highetching resistance of a nitride compound semiconductor.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide alight-emitting diode having a nitride compound semiconductor, whichfeatures high external quantum efficiency, stable characteristics, andeasy design and processing.

A semiconductor light-emitting device according to the present inventioncomprises: an element having: a semiconductor multilayer film containinga nitride and having an active layer; and a transparent layer providedon the semiconductor multilayer film and having projections/depressionsof a two-dimensional periodic structure at an upper surface thereof todiffract light from the active layer at the projections/depressions andguide the diffracted light to an outside of the semiconductor multilayerfilm.

In the arrangement, the radiation angle of the light relative to theinterface between the transparent layer and an air, which is larger thana critical refractive angle when the light is emitted from the activelayer, becomes smaller than the critical refractive angle. This enableslight that has been totally reflected conventionally to be extracted tothe outside and thereby improves light extraction efficiency.

To cause diffraction, it is sufficient for the projections/depressionsto have a two-dimensional periodic structure. Unlike a diffractiongrating of a one-dimensional periodic structure, the two-dimensionalperiodic structure exerts a diffractive effect on light at a radiationangle in any direction so that it is highly effective in improving thelight extraction efficiency. Since diffraction occurs provided thatprojections/depressions are arranged with regular pitches in verticaland lateral directions, the influence of variations in the configurationand size of the projections/depressions on the light extractionefficiency is reduced compared with the conventional embodiment whichimproves the light extraction efficiency by providing the lens andthereby refracting light. This ensures high light extraction efficiencyeven if the configuration and size of the projections/depressions varyduring the fabrication process, allows easy design and processing, andimproves the fabrication yield.

When a distance between each of depressed portions of theprojections/depressions and the active layer is D and a wavelength ofthe light from the active layer in the element is λ, if D≦5λ issatisfied, the projections/depressions and the active layer are inproximity so that the diffractive effect improves the light extractionefficiency even if the height of each of the projections/depressions isnot so large, which allows easy processing.

When a period of the projections/depressions is L and a wavelength ofthe light from the active layer in the element is λ, if λ≦L≦20λ issatisfied, diffraction occurs effectively to improve the lightextraction efficiency. If λ<1 is satisfied, the angle changed bydiffraction is excessively large so that the light extraction efficiencyis not improved. If λ>20 is satisfied, the light extraction efficiencyis reduced. This is because an excessively long period lowers thediffraction efficiency so that the proportion of a portion of lighthaving its radiation angle changed by diffraction is reduced.

When a height of each of the projections/depressions is h and awavelength of the light from the active layer in the element is λ, ifh≦5λ is satisfied, the projections/depressions to be formed need not beso high so that easy processing is allowed.

The transparent layer may be a first nitride semiconductor layer and anelectrode layer may further be provided on the first nitridesemiconductor layer. In this case, the diffraction efficiency is highand the light extraction efficiency is high because theprojections/depressions are formed directly in the nitride semiconductorlayer with a high refractivity.

In this case, an upper surface of the electrode layer may be providedwith projections/depressions reflecting the projections/depressions ofthe upper surface of the first nitride semiconductor layer.

Alternatively, the electrode layer may be a metal or a metal oxidehaving a film thickness of 50 nm or less.

Alternatively, the electrode layer may be an indium tin oxide.

The semiconductor multilayer film may further have a nitridesemiconductor layer of a first conductivity type provided on the activelayer and a nitride semiconductor layer of a second conductivity typeprovided under the active layer and the transparent layer may have anelectrode layer provided on the nitride semiconductor layer of the firstconductivity type.

In this case, an upper surface of the electrode layer may be providedwith the projections/depressions of a two-dimensional periodicstructure. The arrangement allows easy and uniform injection of thecurrents irrespective of the projections/depressions of the nitridesemiconductor layer of the first conductivity type and prevents areduction in power conversion efficiency due to non-uniform currentinjection.

Alternatively, the transparent layer may further have a layer providedon the electrode layer and having the projections/depressions of atwo-dimensional periodic structure. Since the electrode layer is notprocessed in this structure, it does not incur an adverse effect such asnon-uniform current injection compared with the case where thetransparent electrode is formed with projections/depressions. As aresult, the light extraction efficiency can be improved without loweringthe power conversion efficiency. Since conductivity is not required ofthe transparent layer, a material can be selected with higherflexibility than in the case of forming projections/depressions in thenitride semiconductor layer or in the transparent electrode. By using aneasily processible material for the transparent layer and adopting a lowcost fabrication method, a low cost device is implemented.

When the electrode layer is a metal or a metal oxide having a filmthickness of 50 nm or less, it is possible to establish compatibilitybetween conductivity and transmittance so that high power efficiency isimplemented.

The electrode layer may be an indium tin oxide.

The layer having the projections/depressions of a two-dimensionalperiodic structure may be made of a resin. When the layer having theprojections/depressions of a two-dimensional periodic structure is madeof a resin, the projections/depressions can be formed by pressing. Inthis case, extremely fine projections/depressions can be formed easilywithout using a semiconductor process such as lithography or etching.This reduces the fabrication throughput and cost and implements asemiconductor light-emitting device low in cost and high in efficiency.

When each of projecting portions of the projections/depressions of thetransparent layer has a flat upper surface, easier molding can beperformed.

A method for fabricating a semiconductor light-emitting device accordingto the present invention is a method for fabricating a semiconductorlight-emitting device having a semiconductor multilayer film containinga nitride and having an active layer and a transparent layer provided onthe semiconductor multilayer film, the method comprising the steps of:(a) forming the transparent layer on the semiconductor multilayer film;and (b) after the step (a), pressing an upper surface of the transparentlayer against a mold die having a surface provided withprojections/depressions of a two-dimensional periodic structure andthereby forming, at the upper surface of the transparent layer,projections/depressions in inverted relation to theprojections/depressions of the mold die.

The arrangement allows easy formation of extremely fineprojections/depressions with a two-dimensional period without using asemiconductor process such as lithography or etching. This reduces thefabrication throughput and cost and implements a semiconductorlight-emitting device low in cost and high in efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view showing a structure of a semiconductorlight-emitting device according to a first embodiment of the presentinvention;

FIGS. 2A to 2D are perspective views and plan views showing specificarrangements of projections/depressions of a two-dimensional periodicstructure;

FIGS. 3A and 3B are plan views showing those of the specificarrangements in which the pitch of the projections/depressions of atwo-dimensional periodic structure is different in different directions;

FIG. 4 is a plan view showing that of the specific arrangements in whichthe pitch of the projections/depressions of a two-dimensional periodicstructure is different in different regions;

FIGS. 5A and 5B are plan views each showing a structure in which theprojections/depressions of a two-dimensional periodic structure areprovided at only one portion of an electrode;

FIGS. 6A and 6B are perspective views showing specific configurations ofprojecting portions arranged in a two-dimensional periodic structure;

FIGS. 7A to 7D are perspective views and plan views showing structuresin which depressed portions are arranged in a two-dimensional periodicstructure;

FIG. 8 is a graph showing the result of theoretically calculating arelationship between the period of projections/depressions and lightextraction efficiency in the first embodiment;

FIG. 9 is a graph showing the result of theoretically calculating therelationship between the period of projections/depressions and the lightextraction efficiency by varying the height of theprojections/depressions in the first embodiment;

FIG. 10 is a graph showing a relationship between the height of theprojections/depressions and the light extraction efficiency in an LED ata light emission wavelength of 450 nm in the first embodiment;

FIG. 11 is a graph showing a relationship between the light extractionefficiency and the distance between an active layer and theprojections/depressions;

FIGS. 12A and 12B are graphs showing the characteristics of asemiconductor light-emitting device according to the first embodiment,of which FIG. 12A shows a current-voltage characteristic and FIG. 12Bshows a current-light output characteristic;

FIG. 13 is a perspective view showing a structure of a semiconductorlight-emitting device according to a second embodiment of the presentinvention;

FIG. 14 is a perspective view showing a structure of a semiconductorlight-emitting device according to a third embodiment of the presentinvention;

FIGS. 15A to 15C are perspective views illustrating the process steps offorming transparent layers by pressing;

FIG. 16 is a perspective view showing a structure of a conventionalnitride compound semiconductor LED disclosed in FIG. 10 of Prior ArtDocument 1; and

FIG. 17 is a perspective view showing a structure of the conventionalnitride compound semiconductor LED disclosed in FIGS. 5A and 5B of PriorArt Document 1.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the drawings, the individual embodiments of the presentinvention will be described herein below in greater detail.

Embodiment 1

FIG. 1 is a perspective view showing a structure of a semiconductorlight-emitting device according to a first embodiment of the presentinvention. As shown in FIG. 1, the semiconductor light-emitting deviceaccording to the present embodiment is an LED comprising: a sapphiresubstrate 1; a non-doped GaN buffer layer (not shown) provided on thesapphire substrate 1 and having a thickness of 30 nm; an n-type GaNlayer 2 provided on the GaN buffer layer, doped with an n-type impurityat a concentration of 2×10¹⁸ cm⁻³, and having a thickness of 2 μm; anInGaN active layer 3 provided on the n-type GaN layer 2, made ofnon-doped In_(0.45)Ga_(0.55)N with a PL (photoluminescence) peakwavelength of 450 nm, and having a thickness of 3 nm; and a p-type GaNlayer 4 doped with a p-type impurity at a concentration of 7×10¹⁷ cm⁻³and having a thickness of 400 nm. These nitride compound semiconductorsare formed by crystal growth such as MOCVD (Metal-Organic Chemical VaporDeposition) or MBE (Molecular Beam Epitaxy). In the presentspecification, “non-doped” indicates that intentional doping has notbeen performed.

A trench 8 for exposing the n-type GaN layer 2 as the bottom surfacethereof has been formed by partly etching away the InGaN active layer 3and the p-type GaN layer 4. The trench 8 is formed by, e.g., forming aresist mask (not shown) having an opening over the p-type GaN layer 4 byphotolithography and then partly removing the p-type GaN layer 4 and theInGaN active layer 3 by using a dry etching technology such as RIE(Reactive Ion Etching) or ion milling, a photochemical etchingtechnology involving the radiation of ultraviolet light, or a wetetching technology using a heated acid/alkaline solution.

Projections/depressions of a two-dimensional periodic structure areformed at the upper surface of the p-type GaN layer 4. The period of theprojections/depressions, i.e., the spacing between the respectivecenters of the adjacent depressed or projecting portions in a verticalor lateral direction in a two-dimensional plane is 1 μm and the heightof each of the projections/depressions is 200 nm. Theseprojections/depressions are formed by forming a resist (not shown) onthe p-type GaN layer 4, patterning the resist in accordance with thearrangement of the projections/depressions of the two-dimensionalperiodic structure by a method or system such as interference exposure,electron beam exposure, or a stepper (step-and-repeat photolithographicsystem with demagnification), and then performing dry etching or wetetching using the resist as a mask.

As a transparent electrode 5, an ITO (Indium Tin Oxide), e.g., isdeposited to a thickness of 100 nm on the p-type GaN layer 4 formed withthe projections/depressions. The ITO can be formed by sputtering or thelike. For the transparent electrode 5, not only a conductive metal oxidesuch as the ITO but also a metal thin film high in conductivity and lowin contact resistance can be used provided that it shows a hightransmittance to the light emission wavelength of the LED. In that case,the metal thin film preferably has a thickness of 50 nm or less toretain a high transmittance. For the transparent electrode 5, amultilayer film composed of, e.g., Ni with a film thickness of 10 nm andAu with a film thickness of 40 nm provided thereon can be used.

A Ti/Al n-side electrode 6 is provided on the portion of the n-type GaNlayer 2 exposed as the bottom surface of the trench 8. Further, an Aup-side bonding electrode 7 is provided on the ITO transparent electrode5 provided on the region of the p-type GaN layer 4 unformed withprojections/depressions.

Although the upper surface of the transparent electrode 5 is providedwith the projections/depressions reflecting the projections/depressionsof the upper surface of the p-type GaN layer 4 (transparent layer) inFIG. 1, it is also possible to provide the upper surface of the p-typeGaN layer 4 with projections/depressions and provide a flat transparentelectrode 5 thereon in the present embodiment.

In the semiconductor light-emitting device according to the presentembodiment, the projections/depressions with a two-dimensional periodare formed at the surface of the p-type GaN layer 4 so that the lightfrom the active layer 3 is diffracted. As a result of diffraction, theradiation angle of the light relative to the interface between thesemiconductor and the air, which is larger than the critical refractiveangle when the light is emitted from the active layer, becomes smallerthan the critical refractive angle. In contrast to the conventionalembodiment in which the light forming an angle larger than the criticalrefractive angle between itself and a normal to the interface betweenthe semiconductor light-emitting device and the air is totallyreflected, such light is diffracted in the present embodiment to beextracted to the outside of the LED so that the light extractionefficiency is improved. The present invention is characterized by atwo-dimensional periodic structure, which is different from adiffraction grating in which projections/depressions have a periodicstructure only in one direction. Since the projections/depressions ofthe two-dimensional periodic structure exert a diffractive effect onlight radiated in any direction, they are highly effective in improvingthe light extraction efficiency.

A description will be given herein below to the specific types of thetwo-dimensional periodic structure with reference to the drawings. FIGS.2A to 7D are perspective views and plan views showing the specific typesof the two-dimensional periodic structure, of which FIG. 2A shows thecase where projecting portions 11 are arranged in a triangular latticeand FIG. 2C shows the case where the projecting portions 11 are arrangedin a tetragonal lattice. FIGS. 2B and 2D show respective structuresobtained when these projecting portions 11 are actually formed on theupper surfaces of the transparent electrodes 5. The projecting portions11 may also have different periods in different directions.Specifically, the projecting portions 11 may be arranged in a triangularor tetragonal lattice in which the pitch of the adjacent projectingportions 11 is different in different directions, as shown in FIGS. 3Aand 3B. The period of the projecting portions 11 may also be differentin different regions. Specifically, the projecting portions may beprovided in a state denser at the center portion of the transparentelectrode 5 and in a state less dense at the upper and lower portionsthereof than at the center portion, as shown in FIG. 4. Theprojections/depressions may be provided at only one portion of thetransparent electrode 5. Specifically, the projecting portions 11 may bearranged to be rotationally symmetrical relative to the center portionof the transparent electrode 5, as shown in FIG. 5A. Alternatively, theprojecting portions 11 may be provided only at the center of thetransparent electrode 5 and the portion surrounding the center portionmay be flat, as shown in FIG. 5B. The projections/depressions may alsohave configurations other than cylindrical columns. Specifically, theprojecting portions 11 may be tetragonal columns or hexagonal columns,as shown in FIGS. 6A and 6B. In each of the foregoing structures,depressed portions may be arranged with a two-dimensional period insteadof the projecting portions. Specifically, depressed portions 12 may bearranged in a triangular lattice, as shown in FIG. 7A, or in atetragonal lattice, as shown in FIG. 7C. FIGS. 7B and 7D show respectivestructures obtained when these depressed portions 12 are actually formedin the upper surfaces of the transparent electrodes 5.

A description will be given herein below to a relationship between theperiod of the projections/depressions and the light extractionefficiency with reference to FIG. 8. FIG. 8 is a graph showing theresult of theoretically calculating the relationship between the periodof the projections/depressions and the light extraction efficiency inthe first embodiment. The period of the projections/depressionsindicates the spacing between the respective centers of those of theprojections or depressions equally spaced in vertical and lateraldirections which are adjacent in a vertical or lateral direction in atwo-dimensional plane. In the graph, the abscissa represents the periodof the projections/depressions normalized to a wavelength in the LED ata light emission wavelength of 450 nm, i.e., a relative value of theperiod of the projections/depressions to the wavelength in the LED andthe ordinate represents light extraction efficiency normalized to avalue when the projections/depressions are not formed. Here, thewavelength in the LED is a value obtained by dividing the light emissionwavelength in vacuum or in an atmosphere by the refractivity of asemiconductor. When the light emission wavelength is 450 nm and therefractivity of the semiconductor is 2.5, the wavelength is the LEDbecomes 180 nm.

From FIG. 8, it can be seen that the light extraction efficiency hasincreased while the period of the projections/depressions is in therange of 1 to 20 times the wavelength in the LED. Since theprojections/depressions are formed directly in the nitride compoundsemiconductor layer having refractivity as high as 2.5, high diffractionefficiency is achieved so that the light extraction efficiency hasimproved greatly to four times the conventional light extractionefficiency at the maximum.

When the period of the projections/depressions is equal to or less thanthe wavelength in the LED, diffraction causes an excessively largechange in angle, which leads to a post-diffraction radiation anglelarger than the critical refractive angle, so that the light extractionefficiency does not improve. When the period of theprojections/depressions is twenty times the wavelength in the LED ormore also, the excessively long period lowers diffraction efficiency andreduces the proportion of a portion of light having its radiation anglechanged by diffraction so that the effect of improving light extractionefficiency lowers disadvantageously.

To thus cause diffraction, it is sufficient for theprojections/depressions to have a two-dimensional periodic structure. Inshort, it is sufficient for the projections/depressions to have regularpitches in vertical and lateral directions. Compared with theconventional embodiment which refracts light with the provision of thelens and thereby improves the light extraction efficiency, variations inthe configuration and size of the projections/depressions have lessinfluence on the light extraction efficiency in the present embodiment.This ensures high light extraction efficiency even when theconfiguration and size of the projections/depressions vary during thefabrication process so that the present embodiment facilitates designand processing and improves a fabrication yield.

A description will be given herein below to the result of theoreticallycalculating a relationship between the height of theprojections/depressions and the light extraction efficiency withreference to FIGS. 9 to 11. FIG. 9 is a graph showing the result oftheoretically calculating the relationship between the period of theprojections/depressions and the light extraction efficiency by varyingthe height of the projections/depressions in the first embodiment. Inthe graph, the abscissa represents a relative value of the period of theprojections/depressions to the wavelength in the LED and the ordinaterepresents the light extraction efficiency normalized to the value whenthe projections/depressions are not formed. Four types of theoreticalcalculations using the different heights of the projections/depressionswere performed and the resulting values are shown as different profiles.It is to be noted that the relative values of the heights of theprojections/depressions in FIG. 9 are the heights of theprojections/depressions relative to the wavelength in the LED. FIG. 10is also a graph showing the relationship between the heights of theprojections/depressions in the LED at the light emission wavelength of450 nm and the light extraction efficiency, similarly to FIG. 9.However, in contrast to FIG. 9 which shows the period and heights of theprojections/depressions as relative values, FIG. 10 shows the period andheights of the projections/depressions as specific values. FIG. 11 is agraph showing a relationship between the light extraction efficiency anda distance between the active layer and the projections/depressions. InFIG. 11, the abscissa represents a distance between the depressedportions of the projections/depressions and the active layer and theordinate represents the light extraction efficiency.

As can be seen from FIG. 11, the light extraction efficiency increasesas the distance between the projections/depressions and the active layerdecreases. Since the light extraction efficiency is higher when thedistance between the active layer and the projections/depressions isabout 0.9 μm or less in FIG. 11, it may be said that the distancebetween the active layer and the projections/depressions is preferablyfive times (180 nm) the wavelength in the LED or less. In the case wherethe distance between the active layer and the projections/depressions isthus small, the light extraction efficiency becomes double theconventional light extraction efficiency or more even when the height ofthe projections/depressions is as small as about 1 times the wavelengthin the LED (about 180 nm). This is because the active layer and theprojections/depressions are in proximity so that theprojections/depressions need not be so high. Thus, it is sufficient forthe projections/depressions necessary for diffracting light in thepresent embodiment to have a depth smaller than that of the conventionalprojections/depressions which refract light with the provision of thelens and thereby improve the light extraction efficiency. Since thedepth of the projections/depressions formed in a nitride compoundsemiconductor, which is difficult to process, can be reduced in thepresent embodiment, processing is easier than in the conventionalembodiment and fabrication cost can also be reduced.

The characteristics of the semiconductor light-emitting device accordingto the present embodiment will be described with reference to FIGS. 12Aand 12B. FIGS. 12A and 12B are graphs showing the characteristics of thesemiconductor light-emitting device according to the first embodiment,of which FIG. 12A shows the current-voltage characteristic and FIG. 12Bshows the current-light output characteristic. Each of the graphs alsoshows, for comparison, the characteristics of a semiconductor device ofa conventional structure in which projections/depressions are not formedat the surface of the p-type GaN layer 4 (having a film thickness of 200nm).

From the current-voltage characteristic of FIG. 12A, it can be seen thatthe rising voltage of the semiconductor light-emitting device accordingto the present embodiment shows substantially the same curve as that ofthe conventional embodiment. This indicates that, in spite of theprojections/depressions as shallow as 200 nm provided at the surface ofthe p-type GaN layer 4 of the semiconductor light-emitting deviceaccording to the present embodiment, the current-voltage characteristicthereof has not been adversely affected thereby compared with that ofthe conventional embodiment in which projections/depressions are notformed.

From the current-light output characteristic of FIG. 12B, it can be seenthat a light output obtained with the same current from the deviceaccording to the present embodiment has increased to 3.5 times the valueobtained from the conventional embodiment, which is substantially thesame as the result of the theoretical calculation shown in FIG. 8. Thismay be because a reduction in internal quantum efficiency and areduction in power conversion efficiency each resulting from theformation of projections/depressions are circumvented in thesemiconductor light-emitting device according to the present embodiment.

That is, since the present embodiment has formed theprojections/depressions only in the p-type GaN layer (contact layer) ata distance from the active layer, an increase in hole-electron surfacerecombination caused by the projections/depressions can be circumventedcompared with the case where the projections/depressions are formed evenin the active layer. This prevents a reduction in internal quantumefficiency (the proportion of currents injected into the LED andconverted to light in the LED to all the currents injected into theLED). By forming the transparent electrode 5 entirely over theprojections/depressions, the currents can be injected uniformly into thep-type GaN layer irrespective of the projections/depressions of thep-type GaN layer. This prevents a reduction in power conversionefficiency resulting from non-uniform injection of the currents. Thus,the present embodiment can provide the semiconductor light-emittingdevice high in light extraction efficiency without lowering the currentcharacteristics and with high mass producibility.

Embodiment 2

FIG. 13 is a perspective view showing a structure of a semiconductorlight-emitting device according to a second embodiment of the presentinvention. As shown in FIG. 13, the semiconductor light-emitting deviceaccording to the present embodiment is an LED comprising: a sapphiresubstrate 1; a non-doped GaN buffer layer (not shown) provided on thesapphire substrate 1 and having a thickness of 30 nm; an n-type GaNlayer 2 provided on the GaN buffer layer, doped with an n-type impurityat a concentration of 2×10¹⁸ cm⁻³, and having a thickness of 2 μm; anInGaN active layer 3 provided on the n-type GaN layer 2, made ofnon-doped In_(0.45)Ga_(0.55)N with a PL peak wavelength of 450 nm, andhaving a thickness of 3 nm; and a p-type GaN layer 4 provided on theInGaN active layer 3, doped with a p-type impurity at a concentration of7×10¹⁷ cm³, and having a thickness of 200 nm.

A trench 8 for exposing the n-type GaN layer as the upper surfacethereof has been formed by partly etching away the InGaN active layer 3and the p-type GaN layer 4.

As a transparent electrode 5, a transparent metal oxide havingconductivity such as ITO has been provided to a thickness of 300 nm onthe p-type GaN layer 4. The transparent electrode 5 has been formed onthe p-type GaN layer 4 by sputtering or CVD (Chemical Vapor Deposition).The upper surface of the transparent electrode 5 is formed withprojections/depressions with a period of 0.5 μm and a height of 200 nmwhich are arranged in a two-dimensional periodic structure. Theseprojections/depressions are produced by the following method. First, ametal oxide film (not shown) having a flat upper surface is deposited onthe p-type GaN layer 4. Then, a resist is formed on the metal oxide filmand patterned by a method or system such as interference exposure,electron beam exposure, or a stepper such that a resist pattern for thearrangement of the projections/depressions of a two-dimensional periodicstructure is formed on the metal oxide. In this state, dry etching suchas RIE or ion milling or wet etching using an acid such as HCl isperformed to remove the portions of the metal oxide film uncovered withthe resist pattern and thereby form the transparent electrode 5 havingthe projections/depressions at the upper surface thereof.

A Ti/Al n-side electrode 6 is disposed on the portion of the n-type GaNlayer 2 exposed as the bottom surface of the trench 8. Further, an Aup-side bonding electrode 7 is formed on the region of the transparentelectrode 5 unformed with the projections/depressions.

The present embodiment is characterized in that the p-type GaN layer 4has a flat upper surface and the projections/depressions with atwo-dimensional period are formed at the surface of the transparentelectrode 5. When the period of the projections/depressions is in therange of 1 to 20 times the wavelength in the LED, the light extractionefficiency can be increased by the same diffractive effect as describedabove in the first embodiment.

In this structure, the projections/depressions are formed in thetransparent electrode 5 made of ITO having a refractivity (2.0) lowerthan that of a nitride compound semiconductor so that an increase inlight extraction efficiency is 2.5 times the conventional lightextraction efficiency, which is lower than in the first embodiment.However, since the formation of projections/depressions in a metalcompound such as ITO is easier than the formation ofprojections/depressions in a nitride compound semiconductor layer havinghigh etching resistance, the fabrication cost can be reduced. Incontrast to the case of forming projections/depressions in a nitridecompound semiconductor where device design should be performed withcaution by, e.g., providing a distance of about 0.1 μm between thebottom portions of the projections/depressions and the active layer toavoid the lowering of current characteristics, the present embodimentcan use the conventional device design without alternations since thesemiconductor multilayer film structure of the present embodiment is thesame as that of the conventional device.

Embodiment 3

FIG. 14 is a perspective view showing a structure of a semiconductorlight-emitting device according to a third embodiment of the presentinvention. As shown in FIG. 14, the semiconductor light-emitting deviceaccording to the present embodiment is an LED comprising: a sapphiresubstrate 1; a non-doped GaN buffer layer (not shown) provided on thesapphire substrate 1 and having a thickness of 30 nm; an n-type GaNlayer 2 provided on the GaN buffer layer, doped with an n-type impurityat a concentration of 2×10¹⁸ cm⁻³, and having a thickness of 2 μm; anInGaN active layer 3 provided on the n-type GaN layer 2, made ofnon-doped In_(0.45)Ga_(0.55)N at a PL peak wavelength of 450 nm, andhaving a thickness of 3 nm; and a p-type GaN layer 4 provided on theInGaN active layer 3, doped with a p-type impurity at a concentration of7×10¹⁷ cm⁻³, and having a thickness of 200 nm. These nitride compoundsemiconductors are formed by crystal growth such as MOCVD or MBE.

A trench 8 for exposing the n-type GaN layer 2 as the bottom surfacethereof has been formed by partly etching away the InGaN active layer 3and the p-type GaN layer 4. As a transparent electrode 5, a filmobtained by successively stacking Ni with a film thickness of 10 nm andAu with a film thickness of 40 nm in layers is provided on the p-typeGaN layer 4. A Ti/Al n-side electrode 6 is provided on the portion ofthe n-type GaN layer 2 exposed as the bottom surface of the trench 8.

A plurality of transparent layers 9 each made of a transparent materialsuch as a resin or a metal oxide are arranged in a two-dimensionalperiodic structure on the upper surface of the transparent electrode 5.The period with which the transparent layers 9 are arranged is 1.5 μmand the height of the transparent layers 9 from the transparentelectrode 5 is 300 nm. The present embodiment has used a polycarbonateresin as the material of the transparent layers 9. Further, an Au p-sidebonding electrode 7 has been formed on a part of the transparentelectrode 5.

When the period of projections/depressions provided by the transparentlayers 9 at the surface of the semiconductor light-emitting device is inthe range of 1 to 20 times the wavelength in the LED, light extractionefficiency can be increased by the same diffractive effect as achievedin the first and second embodiments described above.

In this structure, the projections/depressions are formed by using aresin with a refractivity (about 1.5) or a metal oxide with arefractivity (about 2.0) lower than that of a nitride compoundsemiconductor so that an increase in light extraction efficiency is 2.0to 2.5 times the conventional light extraction efficiency, which islower than in the first embodiment. However, since it is easier todispose a resin or a metal oxide in a two-dimensional periodic structurethan to partly etch away a nitride compound semiconductor layer havinghigh etching resistance, the fabrication cost can be reduced. Comparedwith the case where the projections/depressions are formed in thenitride compound semiconductor or in the transparent electrode, thepossibility of giving an adverse effect such as non-uniform currentinjection is reduced. This improves light extraction efficiency withoutlowering power conversion efficiency.

Since the transparent layers 8 are formed on the transparent electrode5, conductivity is not required of the transparent layers 8. As aresult, a material can be selected with higher flexibility and an easilyprocessible material can be used for the transparent layers 8. If aresin is used for the transparent layers 8, the transparent layers 8 canbe formed by transferring the projections/depressions by pressing astamp preliminarily formed with the projections/depressions against theheated resin without using a semiconductor processing technique such asphotolithography or etching. FIGS. 15A to 15C are perspective viewsillustrating the process steps of forming the transparent layers bypressing. In FIGS. 15A to 15C is shown the case where depressed portionsare arranged in the transparent layers, not the case where thetransparent layers 8 are provided in projecting relation. In the stepshown in FIG. 15A, a mold die 21 having projections/depressions and alight-emitting diode 20 provided with a transparent layer 22 having aflat upper surface are prepared first. Then, in the step shown in FIG.15B, projections/depressions in inverted relation to theprojections/depressions of the mold die 21 are reflected in thetransparent layer 22 by joining the surface of the mold die 21 formedwith the projections/depressions and the upper surface of thetransparent layer 22 and pressing the mold die 21 from above at atemperature which softens the resin or the like composing thetransparent layer 22. When the mold die 21 is removed from thetransparent layer 22 in the step shown in FIG. 15C, depressed portions23 arranged in the two-dimensional periodic structure are provided inthe surface of the transparent layer 22. Since the method which formsthe projections/depressions by pressing allows extremely low costformation of a microstructure, unlike the foregoing semiconductortechniques such as photolithography and etching, a semiconductorlight-emitting device low in cost and high in light extractionefficiency can be fabricated.

Thus, the present embodiment can provide a semiconductor light-emittingdevice high in light extraction efficiency with high mass producibility.

Although each of the foregoing embodiments has particularly describedthe case using the nitride compound semiconductor which is difficult toprocess, the design of the present invention is also applicable to aninfrared or red semiconductor light-emitting device using AlGaAs (havinga refractivity of 3.6) or AlGaInP (having a refractivity of 3.5) as asemiconductor. The wavelength in the LED becomes about 240 nm in thecase of infrared light at an oscillation wavelength of 850 nm andbecomes about 180 nm in the case of red light at an oscillationwavelength of 620 nm. Accordingly, it can be seen from FIG. 8 that theeffect of improving the light extraction efficiency becomes maximum whenthe period of the projections/depressions is 1.6 μm in the case of theinfrared light and 1.2 μm in the case of the red light. Thus,micrometer-order processing can be implemented at lower cost thansubmicrometer-order processing.

1-16. (canceled)
 17. A method for fabricating a semiconductorlight-emitting device having a semiconductor multilayer film containinga nitride and having an active layer and a transparent layer provided onthe semiconductor multilayer film, the method comprising the steps of:(a) forming the transparent layer on the semiconductor multilayer film;and (b) after the step (a), pressing an upper surface of the transparentlayer against a mold die having a surface provided withprojections/depressions of a two-dimensional periodic structure andthereby forming, at the upper surface of the transparent layer,projections/depressions in inverted relation to theprojections/depressions of the mold die.